To obtain a bright and compact large aperture ratio objective lens suited to a dark field optical device and capable of generating a large negative distortion aberration by forming telescopic constitution consisting of seven lenses of three positive, negative and positive groups arranged successively from the object side and satisfying prescribed conditional expressions.

A 1st lens group G1 is composed of two positive lenses L1, L2, and of a cemented lens consisting of a negative lens L3 and a positive lens L4 arranged in order from the object side, a 2nd lens group G2 consists of a negative lens L5, a 3rd lens group G3 is composed of a cemented lens consisting of positive lens L6 and a negative lens L7, and the most object side face of the 3rd lens group G3 is formed convex toward the object side. Conditional expressions 0.45<FG1/f<0.9 (1), 0.3<fG2/f<0.7 (2), 0.15<RG31/f<0.6 (3), and 0.4<dG2G3/f<0.8 (4) are satisfied. Provided that f is the focal distance of the whole system, fG1 is the focal distance of the 1st lens group G1, fG2 is the focal distance of the 2nd lens group G2, RG31 is the radius of curvature of the most object side face of the 3rd lens group G3, and dG2G3 is an interval between the 2nd lens group G2 and the 3rd lens group G3.